improve MapIndexMaxedOut error logging

Summary:
The current exception string for MapIndexMaxedOut is not very useful because ```oldHashTable.viewWriteHandle()->getKey()``` can have null terminators in it and cause the c_str() to be terminated incorrectly. Regardless, the actual write handle key is not very useful to us. Log the aid explicitly.

Example log:
```
Sep 14 07:12:57 tsp_lla/tao/prod-opt-follower/1879 dpdaemon[442]: E0914 07:12:57.836133  1922 Refill.cpp:215] Index has maxed out for key: pmem#1<A4><D1 (https://github.com/facebook/CacheLib/commit/3ec18f5591f75fa5e247cadad19a86126075bdda)>0F<F6>\
```

Reviewed By: vrishal

Differential Revision: D39518530

fbshipit-source-id: 58b9f31f6c0d174d5ec0bc7705788408bc0d11f4

Author: Evan Liang

cachelib/datatype/Map-inl.h

@@ -208,10 +208,9 @@ auto copyHashTable(
   if (newSize > kMaxHashTableSize) {
     // This shouldn't happen. Too many entries for a single object in memory.
     auto exStr = folly::sformat(
-        "Index has maxed out for key: {}. Existing entries: {}, New requested "
-        "capacity: {}. New requested size: {}",
-        oldHashTable.viewWriteHandle()->getKey(), oldHashTable->numEntries(),
-        newCapacity, newSize);
+        "Index has maxed out for the provided key. Existing entries: {}, New "
+        "requested capacity: {}. New requested size: {}",
+        oldHashTable->numEntries(), newCapacity, newSize);
     throw cachelib::MapIndexMaxedOut(exStr.c_str());
   }